Free piston machine the compressor portion of which includes at least two stages



Jan. 26, 1954 R. HUBER v 2,667,300

FREE `PISTON MACHINE THE COMPRESSOR PORTION OE wHIcH INCLUDES AI LEAST Iwo STAGES Filed June 9, 1950 INVENTUR RBEQT /asf/ p ATTORN @MUS Y s n mv .U S E WS ,t Q I mnm|k\|\\l ENWHWHIMQ m E n Nl m H I m. n m Y Vf v V \N. Q h- EN Y Ns# Etlik. wh "Hw w EN E- .WNN il A N. E t I w U Y l u l f l I a l 1 0 l Amm EYS

Patented Jan. 26, 1954 FREE PISTON MACHINE THE COMPRESSOR PORTION F WHICH INCLUDES AT LEAST TWO STAGES Robert Huber, Bellevue, France, assignor to Societe dEtudes et de Participations, Eau, Gaz, Electricite, Energie, S. A., Geneva, Switzerland,

a society of Switzerland Application June 9, 195o, serial No. 167,062 Claims priority, application France June 14, 1949 Claims.

The invention relates to free piston machines the compressor part of which includes at least two stages.

These machines may be either motor-compressors serving to supply compressed air or autogenerators serving to supply a mixture of compressed air and incompletely expanded combustion gas. Furthermore, these machines may be fitted with either simple piston systems or double opposed piston systems.

One of the characteristics of free piston machines of the above mentioned type consists in the possibility of varying their output per stroke by varying the amplitude of the stroke of their free piston systems.

When these machines work on no load, they have the minimum length of stroke, which is that for which the compressor piston or pistons achieve just the compression of the gas, in particular air, to be compressed, without delivering it to the outside of the compressor cylinder. Consequently the output for this length of stroke is zero. The more the length of stroke exceeds this minimum value, the greater the amount of air compressed Yand discharged from the compressor cylinder, and this amount reaches its maximum value for the maximum length of stroke compatible with safe operation of the machine, which maximum length of stroke depends upon the dimensions of the elements of said machine.

The distributing means of the power cylinder of such a machine are generally controlled by the free piston system or systems. As a rule, the power piston or pistons of these systems directly control the exhaust and intake ports of the power cylinder, in particular when the power part of the free piston machine works on the two-stroke cycle. Of course, a suiiicient opening of these distribution means is to be achieved for all possible lengths of stroke of the machine, including the minimum length of stroke which corresponds to the machine running on no load.

In order to give the power part of the machine, which is of given dimensions, as high a power as possible, and in order to take the highest advantage of the combustion energy by a prolonged expansion of the combustion gases, it is of interest to open the intake and exhaust orices, during the working stroke (outward stroke) of the power pistons, only after as long as possible a movement of the power piston or pistons in the outward direction.

When said piston or pistons directly control the intake and exhaust orices, it is therefore of interest to dispose these orifices at the maximum possible distance from the inner dead center position of the piston or pistons. In order to achieve this result while complying with the condition above stated, to wit that, even when running on no load (minimum stroke of the machine) the intake and exhaust orices must besuciently open, it is necessary to reduce as much as possible the difference between the minimum stroke and the maximum stroke. However, in known machines, this diiierence is relatively great, when the compressor part of the machine includes several stages having respectively the same ratio ofk compression, i. e. the same ratio of swept volume plus clearance volume to clearance volume.

According to my invention, in order to obtain a reduction of this difference in a free pistonA machine having two or more stages of compression, the ratio of compression in the iirst compression stage is made higher than that obtained in the second stage and, possibly, in the following stages, and I provide means for ensuring simultaneity of the beginning of discharge in all stages.

Preferred embodiments of my invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example and in which:

Fig. 1 diagrammatically shows, in longitudinal section, a motor compressor having opposed free pistons, made according to the invention;

Fig. 2 is an explanatory diagram;

Fig. 3 is a view similar to Fig. l of a modied form of the invention;

Fig, 4 is an explanatory diagram of Fig. 3.

The compressor includes a power cylinder I in which work two power pistons 2 and 3 having opposed movements. Cylinder l is provided, in its middle part, with a fuel injection device 4 and, on either side of this device, at a distance therefrom, with intake ports 5 and exhaust ports 6. Each of the power pistons is connected with one or several compressor pistons which work either in the compressor part of the machine or in a return energy accumulator.

In the embodiment shown by Fig. 1, power piston 2 is rigid with a piston I working in the cylinder 8 of the return energy accumulator device, pistons 2 and 'l together with their connecting rod 9 constituting one of the moving units of the machine.

Power piston 3 is connected, through a rod I0, with two compressor pistons Il and I2, the iirst of which, to wit Il, is the low pressure stage compressor piston, whereas piston I2 is the high pressure stage compressor piston.

Pistons 3, II and I2, together with their connecting rod I0, constitute the second free piston moving unit.

The two moving units thus constituted are connected together by a synchronizing device of known construction, not shown on the drawing. This synchronizing device causes these units to move in opposite directions and to have, at any time, respective positions which are symmetrical' with respect to the middle plane passing through the center of power cylinder I.

Low pressure piston I I works in a low pressure compressor cylinder I3 provided, on one side of piston II, with a suction valveV I4 and a discharge valve I for the air to be compressed and delivered to the outside, whereas, on the other side of piston II, another suction valve I6 and another discharge valve II-.are provided, these last mentioned valves transforming the space of cylinder I3 in which they are provided into a scavenging and supercharging pump for power cylinder I. The air discharged through the valve I1 of this pump is stored up in the casing I8 of the machine, which casing surrounds motor cylinder I and is separated from the inside of cylinder I3 by partition I9, in which valve II is mounted. The high pressure piston I2 works in a high pressure compressor cylinder 20 tted with an inlet valve 2| and a discharge valve 22, and an intermediate reservoir 23 is provided between the discharge valve I5 of the low pressure cylinder I3 and the chamber 24.

Concerning the arrangement of intake port 5 and exhaust port B, their distance e from the center 0 of the power cylinder is given, as above explained, by the minimum length of stroke of the moving units of the machine; in other words, for this length of stroke, the intake and exhaust strokes must be sufliciently opened by pistons 2 and 3 to ensure scavenging and lling of the power cylinder. If, for a given ratio of the total compression in the compressor, I made use not of a two-stage compressor but of a single stage compressor, the difference between the lengths of stroke for running on no load, on the one hand, and running `on full load, on the other hand, would be minimum and, consequently, the value to be given to distance e would be minimum as shown by Fig. 1.

As a matter of fact, in Fig. 2 I have shown by A-B-C-D the pressure-length of stroke diagram of a single stage compressor, when this compressor is running on full load. For this full load, discharge takes place during period B--C andthe value of the length of stroke is a.

When the compressor is running on no load, the diagram shrinks into line A-B, i. e. the stroke in the compressor cylinder is Vjust suiilcient to reach, at the end of the compression time, the discharge pressure p2., without however air being actually discharged from the compressor cylinder. The length of stroke for running on no load, in the case of a single stage compressor is therefore equal to b and it is this value b which determines, in the case that is `being considered, the location of orices 5 and 5.

4 I-I-K-C-L in the high pressure stage. It will be seen that, in this case, the diierence between the maximum length of stroke, which is still a, and the minimum stroke, which is now d and corresponds to the running on no load of the two stage compressor having the same ratio of compression, increased very much with respect to the case of the single stage compressor. In order to have, in this last mentioned case, ports 5 and 6 suiiiciently uncovered during the running on no load of the compressor, difference e must be given a value corresponding to length of stroke d. The power and eiciency of the power portion of the machine are therefore considerably reduced for the'same dimensions.

In order to avoid these drawbacks, which result from the replacement of the single stage compressor portion by a two stage compressor `'portion having the same ratio of compression, I

arrange, according `to my invention, the two higher than that of the second stage and, further, that discharge' begins simultaneously in both stages. In order to achieve this simultaneity, I may either increase the clearance space of the high pressure cylinder or delay the beginning of compression in the high pressure cylinder. It is this last mentioned solution which is adopted in the machine shown by Fig. l, where the high pressure cylinder 20 is located in a chamber 24 sure of the high pressure cylinder and which communicates with cylinder 20 through ports 25 provided in the wall of said cylinder 20 at the place where compressing is to start therein.

In other words, compressing starts in the high pressure cylinder only when piston I2, in the course of its outward stroke, has closed or moved beyond ports 25.

The machine according to Fig. 1 is for instance arranged in such manner that the ratio of compression in the low pressure compressor cylinder is equal to 4/1, while the ratio of compression .f in the high pressure cylinder is equal to only 2/1. The ratio of compression in the low pressure cylinder is therefore equal to twice the ratio of compression in the high pressure cylinder.

The compression diagram in the low pressure cylinder is indicated by lines A-M-N-O of Fig. 2. The difference between the maximum length of stroke, which remains equal to a and the minimum stroke, which corresponds to running on no load, is now much smaller than in the case of the two stage compressor having the same ratio of compression for both stages. Value c is very close to the optimum value b and the location of ports 5 and 5 may correspond tothe value c of the minimum length of stroke.

The diagram in the high pressure cylinder is determined by lines P-Q-R-C and T. The location of point Q which corresponds to the position of ports 25 is chosen in such manner that the beginning of discharge from the high pressure cylinder, which beginning is indicated by point R, takes place at the same time as the beginning of discharge'in the low pressure cylinder, this last mentioned beginning being indicated by point M.

My invention further has the advantage of subjecting the exhaust valves of the two stages to temperatures which are approximately equal. This is so because the temperature of the air entering the high pressure cylinder is substantially higher than the temperature at the intake 'of the lowpressure cylinder,'especially in a compressor the'cooling agent of which is the surrounding air or Water circulating in a closed circuit and cooled by air. Despite this difference between the temperatures at the respective inlets of the two stages, I obtain, owing to my invention, nearly the same temperatures at their outlets. This is due to the fact that, in the above mentioned example, if it is supposed that air enters the first stage at a temperature of about 20, this air will be, at the outlet of this first stage, at a temperature of about 162 C. The intake temperature in the second stage is higher than the temperature of the cooling water which cools down air in intermediate reservoir 23, so that the air temperature at the inlet of the high pressure cylinder may be reckoned as about 80. With this intake temperature, the outlet temperature in the high pressure stage is about 157 C.

Finally, it should further be noted that a good stability of the machine is obtained by subjecting, as indicated for the compressor shown by Fig. l, the rear face of the high pressure compressor piston to the intake pressure of the high pressure stage. Calculation and experience showed that, in a machine such as above described, the return energy increases slightly with the length of stroke. This increase is useful, since I thus obtain, when running under no load, a minimum number of strokes per unit of time and, when running under full load, a maximum number of strokes. In the modication shown in Fig. 3, a similar result is obtained by a different construction. In this form, the clearance space of the high pressure cylinder is greater than that of the low pressure cylinder, as is clear in the figure from the fact that the distance from the piston Il to the right-hand Wall of the low pressure cylinder is substantially less than the distance from the piston I2 tothe righthand wall of the high pressure cylinder 20.

Otherwise, the parts of Fig. 3 correspond to those of Fig. 1 and bear the same reference characters.

Fig. 4 shows the diagrams of compression obtained by the two stages when using the form shown in Fig. 3. The diagram of the rst stage of compression is, of course, the same as in Fig. 2, since the low pressure stages are the same in both modifications.

On the other hand, the high pressure stage diagram of Fig. 4 shows the distances which separate the face of the piston from the end wall of the cylinder when the piston is nearest this end wall by e and e1, respectively, for the low pressure and high pressure cylinders. It will be noted that e1 is seven times e. This diagram shows that the arrangement of Fig. 3 gives an eect corresponding to that of Fig. 1.

It should also be noted that the invention can be applied both to the transformation of existing machines and to the construction of new machines.

My invention can also be applied to partly free piston machines, i. e. machines where only one of the inner and outer dead center positions of the cylinders is variable and the other is xed.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efficient embodiments of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition 6 y and form of thepartswithoutrdepartingirom theV principle of the presentinvention as comprehended within the scope of the accompanying claims. y

What I claim is: i

1. A freeV pistonmachine which comprises a power portion including a cylinder anda piston freely slidable in said cylinder and a `compressor portion including two compression4 stages, each constituted by one cylinder and one piston running therein, the ratio of swept volume `plus clearance volume to clearance volume in the first of these two stages being higher than in the second of these stages, the cylinders of the power portion and of the respective compressor stages being rigid-with one another and the pistons of the power portion and of the respective compressor stages being rigid with one another, an intermediate reservoir in communication with the discharge of the rst stage, means forming a chamber surrounding the second stage compressor cylinder and in communication with said intermediate reservoir, the cylindrical wall of said second stage compressor cylinder being provided with ports located so as to delay the beginning of compression in said cylinder during the compression stroke of its piston so as to make the beginning of discharge simultaneous in both stages.

2. A free piston machine according to claim 1 in which the inactive surface of said second stage compressor piston is constantly under a pressure equal to the intake pressure of said second stage.

3. A free piston machine which comprises, in combination, a power portion including a cylinder and a piston freely slidable in said cylinder, and a compressor portion including two compressor stages, each constituted by one cylinder and one piston running therein, the ratio of swept volume plus clearance volume to clearance volume in the first of these two stages being higher than in the second of these stages, the cylinders of the power portion and of the respective compressor stages being rigid with one another and the pistons of the power portion and of the respective compressor stages being rigid with one another and means in at least one of said compressor Stages for making the beginning of discharge simultaneous in both of said compressor stages.

4. A free piston machine which comprises, in combination, a power portion including a cylinder and a piston freely slidable in said cylinder, and a compressor portion including two compressor stages, each constituted by one cylinder and one piston running therein, the ratio of swept volume plus clearance volume to clearance volume in the rst of these two stages being higher than in the second of these stages, the cylinders of the power portion and of the respective compressor stages being rigid with one another and the pistons of the power portion and of the respective compressor stages being rigid with one another and means in said second compressor stage for delaying the beginning of compression therein so as to make the beginning of discharge simultaneous in both of said compressor stages.

5. A free piston machine which comprises, in combination, a power portion including a cylinder and a piston freely slidable in said cylinder, and a compressor portion including two compressor stages, each constituted by one cylinder and one piston running therein, each compressor stage cylinder having an end wall and a side wall extending imperforate a certain distance from the end wall, the ratio between the product 7 of the distance from-the end-'wall to the piston when the piston is closest tothe end wall multiplied by the average cross section of the cylinder for such distance and the product of the length of such imperforate portion between the end Wall and the position of the piston most remote from the end wall multiplied by the average cross section of the cylinder for such length being greater in the second stage than in the first stage, the cylinders of the power portionand of the respective compressor stages being rigid with. one another and the pistons of the power portionY and ofthe respectiveV compressor stages being rigid with one another.

Number Name Y Y Date 2,113,691 Heller Apr. 12, 1938 2,241,957 Pescara May 13, 1941 2,304,999 Gonzalez Dec. 15, 1942 Neugebauer Oct. 23, 1945 

